EP 1 155 844 A2 has disclosed one basic idea of precisely registered printing of partial color images or color separations in multi-color printing of toner images with the use of an electrophotographic printing machine. In such a printing machine, the printing material passes through successive imaging stations or printing units that are supplied with a toner of a specific color, and the color separations of this color are transferred to the printing material. For color printing, two or more printing units may be used to achieve this; whereas, for black-and-white printing, one such printing unit is sufficient. For multi-color printing, in most cases four printing units are used, these being provided for the colors cyan, magenta, yellow, and black (briefly also referred to as CMYK).
It should be explained that, referring to electrophotographic printing machines, the concepts “imaging station” and “printing unit” are frequently used synonymously, whereby the description of the “printing unit” has mainly been adopted from the field of offset printing. In more specific terms, an imaging station in electrophotographic printing is a zone in the machine, in which the image support, which is to be charged with the desired image pattern, is provided with an image and developed with toner. This developed image is then transferred directly or indirectly to the printing material; at this point, this could already be referred to as “printing”, even though the completion of printing might still take into account a simultaneous or later fusing of the finished printed image to the printing material. Within that meaning, an imaging station mostly corresponds to a printing unit or a part thereof.
Obviously, a printing machine for multi-color printing could also be used for black-and-white printing. Such a use would be particularly desirable, for example, if a print job for printing a brochure required printing of multi-color picture pages as well as black-and-white text pages, in order to be able to print this brochure in a continuous and sorted manner. Until now, this has been uneconomical because a conventional color-printing machine operates more slowly and more expensively compared with a black-and-white printing machine.
In addition, there is the fact that the toner used in a color-printing machine usually is different from that used in a black-and-white printing machine because the two toners must perform different functions. The black toner in the printing machine is required within the framework of color management and color space transformation in order to implement the color space in which the color-printing machine generates its color-printed images, whereas the black toner in the black-and-white printing machine is to generate exclusively black with the best possible saturation, mostly for printing letters. Additionally, the color print is to be mostly glossy and as brilliant as possible, whereas the letters should be dull and matt for better readability and for greater light absorption.